Avoca Tank (Tritton Operations)

Girilambone-type copper

Avoca Tank sits in the Girilambone district, part of the Tritton Operations, and it belongs to a family of copper deposits hosted in the Ordovician Girilambone Group — a thick pile of deformed, low-grade metamorphosed turbidites and mafic rocks.

GSNSW classes it as a structurally controlled high-sulfide base metal deposit with a hydrothermal-metamorphic genesis. That combination matters: the copper was introduced, and then the whole package was deformed and metamorphosed, remobilising sulphides into the structures that were opening at the time.

Massive and disseminated, in the same breath

The mineralisation style is logged as both disseminated and massive, with a lenticular deposit shape. Chalcopyrite and pyrite dominate, with sphalerite, and an oxide overprint of haematite, limonite, magnetite, malachite and manganese oxides.

That oxide list is a reminder that you are looking at a system with a weathered upper part. Malachite green is cheerful and diagnostic and tells you almost nothing about grade at depth.

Gangue is chlorite, silica and magnetite — the magnetite is worth noting, because it means this deposit talks to magnetic surveys.

The problem is the cover

Look at how this deposit has been drilled and the exploration problem becomes obvious. The dominant method by a wide margin is RAB — rotary air blast. Two thirds of the holes here are RAB. Those are fast, cheap, shallow methods, and you use hundreds of them when your problem is not depth but concealment.

Much of the Girilambone district lies under transported cover. The rock does not outcrop, so before anyone can drill a deep diamond hole they have to punch a grid of cheap holes through the cover to find out where the bedrock geochemistry is even interesting. The diamond holes are a small minority, and they came later.